Conventional power transformer windings used in switch mode power supplies are usually either of the cylindrical or sandwich type (FIG. 1). The main flux produced by either winding arrangement is carried by the high permeability (usually E-shaped ferrite) core. Assuming that the current-turns product in each winding is equal (i.e. balanced mmfs, no magnetising current) the leakage flux due to the winding current is as shown (FIG. 1). This condition is approximated to in most conventional designs but applications of the present invention are not restricted to these cases. Referring to the mmf (NI) diagram it is evident that the leakage flux density: EQU B.sub.1 =(.mu..sub.o N.sub.1 I.sub.i)/L.sub.c
is at a maximum in the air gap between windings. The effect of this leakage flux is to induce eddy currents in the windings which become more severe as the flux density increases. Referring to the flux-linkage (N.phi.) diagram, the distribution of the leakage inductance (flux linkage per amp, N.phi./I) is shown. The leakage inductance can be visualised as the solid of revolution of the N.phi. diagram about the central axis.